1. Field of the Invention
This invention relates to a pipeline inspection vehicle, and more specifically to a device for controlling the speed of the vehicle within the pipeline.
2. The Prior Art
A common way of inspecting internal surfaces of oil or gas pipelines is to use a pipeline inspection vehicle which is provided with the necessary instrumentation for inspection of the pipeline and which is usually driven along within the pipeline by oil or gas flow which reacts with resilient scraper cups that are mounted to be around the body of the vehicle and to be in close relation to the pipe internal wall. A driving force is therefore provided by a differential pressure acting across the resilient scraper cups. Variations in the differential pressure can, however, lead to fluctuations in vehicle speeds, with consequent high and unacceptable accelerations and decelerations. Usually the differential variations are caused by changes in wall friction characteristics, welds, junctions, bends and change of inclination of the pipeline.
One form of pipeline inspection vehicle is disclosed in U.S. Pat. No. 3,495,546. This vehicle is provided with a speed control which features a bypass valve to allow gas within the pipeline to bypass the resilient cups and hence control the differential pressure across the cups.
The use of a by-pass system has not proved completely satisfactory as a speed control system, however, because of the delay in the response time of the vehicle when the by-pass valves are operated, i.e., due to the inherent characteristics of oil or gaseous flow. The system is not sensitive enough for accurate inspection purposes.
It is an object of the present invention to provide a speed control system for a pipeline inspection vehicle such that the vehicle is constrained to travel at approximately the average speed of the fluid flowing in the pipeline and to vary only within acceptable limits without violent accelerations or decelerations.
It is conventional to use more than one inspection vehicle in a pipeline inspection system, for example, two or more vehicles can be formed as a train with one of the vehicles providing the traction force, the others carrying the inspection, recording instruments and auxiliary equipment. That is not to say that the tractor vehicle does not carry auxiliary equipment since the location of the equipment is determined primarily by the space and operating requirements. The speed control system as described in this invention may be applied to the tractor vehicle or to a towed vehicle.
A method of carrying out the invention is described below with reference to drawings which illustrate by way of example only one specific embodiment.